====== Thunderbird ======
===== Bigger font size =====
  * Menu app icon > Options/Preferences > General
  * Scroll down to the very bottom
  * click on 'Config Editor' button to open 'about:config'
  * Accept the risk. 

  * In search type: pixels
  * Look for this line: layout.css.devPixelsPerPx
  * Double left click on that line to open the 'Enter string Value' box so you can edit the Value. The default value will be a negative number: -1.0
  * Enter a new positive number: 1.3 seems to work for me, then click on OK.

The effect is immediate, so you can modify that number until it looks ok for you. Image below as guide.